Reproduction in Drosophila is analyzed genetically. A long-term goal is to establish connections between well-defined genotypic variations and the several specific actions performed by the flies during their courtship and mating. This analysis also delves into the neural basis of fixed action patterns, as impinged upon by mutations and genetic mosaicism. For example, the sex chromosomal constitution of particular portions of the CNS, in gynandromorphs, is identified by enzymatic markers and correlated with quantifiable components of sex-specific behavior. Mosaics are also used to determine the tissues directly affected by behavioral mutations that lead to aberrant reproductive behavior. Some of these variants are related to the olfactory control of interactions between courting flies; these behaviors are also studied through the analysis of pheromones produced by flies of various genotypes. Another basic aspect of communication between flies as they court is vision, whose role in these behaviors is assessed with mutations; some of these have very selective effects on the visual system and thus provide unique tools for dissecting the importance of different inputs into the CNS. The central control of reproductive behavior is also studied with mutations that, perhaps unexpectedly, affect courtship and mating in specific ways: these are variants with learning disabilities or mutants that exhibit aberrant circadian rhythms. The former are defective in conditioned courtship, which also has strong connections to the olfactory control of reproduction; the latter have dramatic effects on short-term behavioral oscillations, that is, in specific features of the male's courtship wing vibrations. The neural basic of aberrant learning in courtship, and of the altered song rhythms, is probed using the mosaic principles and techniques analogous to those applied to the "simpler" visual and olfactory aspects of reproduction. A picture that emerges from this multi-faceted genetic attack on these higher behaviors is how interrelated are the sensory and central mechanisms underlying the fly's actions: Not only do sights and smells influence courtship and mating--as expected, but in ways that are precisely definable genetically--but the neural mechanisms controlling conditioning and circadian rhythms are also tightly entwined with those responsible for the fixed action patterns of reproduction.